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.TH SOCKATMARK 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
sockatmark \- determine whether socket is at out-of-band mark
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.PP
.BI "int sockatmark(int " sockfd );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR sockatmark ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.BR sockatmark ()
returns a value indicating whether or not the socket referred
to by the file descriptor
.I sockfd
is at the out-of-band mark.
If the socket is at the mark, then 1 is returned;
if the socket is not at the mark, 0 is returned.
This function does not remove the out-of-band mark.
.SH RETURN VALUE
A successful call to
.BR sockatmark ()
returns 1 if the socket is at the out-of-band mark, or 0 if it is not.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I sockfd
is not a valid file descriptor.
.TP
.B EINVAL
.\" POSIX.1 says ENOTTY for this case
.I sockfd
is not a file descriptor to which
.BR sockatmark ()
can be applied.
.SH VERSIONS
.BR sockatmark ()
was added to glibc in version 2.2.4.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR sockatmark ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
If
.BR sockatmark ()
returns 1, then the out-of-band data can be read using the
.B MSG_OOB
flag of
.BR recv (2).
.PP
Out-of-band data is supported only on some stream socket protocols.
.PP
.BR sockatmark ()
can safely be called from a handler for the
.B SIGURG
signal.
.PP
.BR sockatmark ()
is implemented using the
.B SIOCATMARK
.BR ioctl (2)
operation.
.SH BUGS
Prior to glibc 2.4,
.BR sockatmark ()
did not work.
.SH EXAMPLES
The following code can be used after receipt of a
.B SIGURG
signal to read (and discard) all data up to the mark,
and then read the byte of data at the mark:
.PP
.EX
    char buf[BUF_LEN];
    char oobdata;
    int atmark, s;

    for (;;) {
        atmark = sockatmark(sockfd);
        if (atmark == \-1) {
            perror("sockatmark");
            break;
        }

        if (atmark)
            break;

        s = read(sockfd, buf, BUF_LEN);
        if (s == \-1)
            perror("read");
        if (s <= 0)
            break;
    }

    if (atmark == 1) {
        if (recv(sockfd, &oobdata, 1, MSG_OOB) == \-1) {
            perror("recv");
            ...
        }
    }
.EE
.SH SEE ALSO
.BR fcntl (2),
.BR recv (2),
.BR send (2),
.BR tcp (7)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
